1. Field of the Invention
The invention relates to technology of a leak test method and a leak test apparatus that detects a leak in a space to be inspected in a work, i.e., an inspection space in a work.
2. Description of Related Art
A leak method according to related art involves increasing the pressure in an inspection space in a work and a space in a master chamber, and detecting a leak in the inspection space in the work by detecting a change in differential pressure between the spaces when a increased pressure state is being maintained. With such a leak test method, reaction force is generated in the work when the pressure is increased. Therefore, a cylinder or the like that applies thrust to a jig that seals the work ends up being larger. That is, the equipment for performing the leak test method ends up being larger.
Japanese Utility Model Application Publication No. 60-111249 (JP 60-111249 U) describes technology related to a leak test method that involves reducing the pressure in an inspection space of a work and a space in a reference tank (i.e., a master chamber), and detecting a change in differential pressure between the spaces using a differential pressure detector when the reduced pressure state is being maintained. Such a leak test method that reduces the pressure in the spaces does not generate reaction force in the work, so the equipment for performing the leak test method can be smaller.
FIG. 16 is a view showing the manner in which residual moisture in the inspection space in the work vaporizes. As shown in FIG. 16, there are cases in which moisture (such as coolant for example) used in a step performed prior to leak detection remains in the inspection space in a work that is to undergo leak detection. Residual moisture in the inspection space at atmospheric pressure is shown in FIG. 16. An example of such a case is when detecting a leak in an inspection space in a work in a mass-production environment where works are manufactured by machining.
The pressure in the inspection space in the work is reduced by air (i.e., air molecules and water vapor) being discharged outside, so the humidity decreases according to the degree of pressure reduction. The inspection space when the pressure has been reduced is shown in FIG. 16. That is, the inspection space in the work dries when the pressure is reduced. Leak detection of the inspection space in the work is performed maintaining the reduced pressure state. As a result, the inspection space in the work is kept in a dry state, and vaporization of the residual moisture ends up progressing. This vaporization of the residual moisture causes the water vapor pressure in the inspection space in the work to rise. A rise in the water vapor pressure in the inspection space when a vacuum is maintained is shown in FIG. 16. Therefore, during leak detection, the pressure in the inspection space changes, and the differential pressure also changes.
That is, with the technology described in JP 60-111249 U, when moisture remains in the inspection space in the work, as shown in FIG. 17A, a large change in the differential pressure occurs due to the vaporization of the residual moisture, even though there is no leak in the inspection space in the work. With the technology described in JP 60-111249 U, it is not possible to distinguish between a change in the differential pressure due to vaporization of residual moisture like that shown in FIG. 17A, and a change in the differential pressure when there is a leak in the inspection space in the work and air flows into the inspection space from outside, as shown in FIG. 17B. In FIG. 17B, the flow of air that flows in the inspection space is indicated by an arrow. That is, with the technology described in JP 60-111249 U, there is a possibility that a work in which there is no leak in the inspection space may end up being detected as a work in which there is a leak in the inspection space. That is, there is a possibility that false detection may end up occurring due to a rise in the water vapor pressure.